Electrical submersible pumps are used for high volume well production. A submersible pump assembly of this type has a centrifugal pump with a large number of stages. The pump is driven by an electrical motor located below the pump and protected by a seal section which contains thrust bearings for pump thrust. The electrical motor is an AC motor supplied with power from the surface through a cable. The entire assembly is mounted on tubing within a cased well.
The motor is typically 30 feet or more. A rotor mounted to a shaft rotates within a stationary stator to drive the pump. A number of radial bearings are located along the length of the motor for radially stabilizing the rotor. A thrust bearing for rotatably supporting the weight of the rotor is located at the top or head of the motor. In tandem motor assemblies, a thrust bearing is located at the head of the lower motor. The motor is filled with a dielectric lubricant.
Because of the high electrical power supplied to the stator, a magnetic field is generated which causes voltage to be induced into the shaft. At times, the voltage buildup can create a differential between the shaft and the stator or housing that is sufficiently high to cause an electrical arc between the rotating and nonrotating components of the bearings.